1. Field of the Invention
This invention generally relates to light emitting diode based measurement systems. Certain embodiments relate to a measurement system that includes one or more arrays of light emitting diodes arranged along a flow path of microspheres or other fluorescence emitting samples.
2. Description of the Related Art
Generally, flow cytometers provide measurements of fluorescence intensity of laser excited polystyrene beads or cells as they pass linearly through a flow chamber. However, flow cytometers can also be used to provide measurements of one or more properties of other particles. Some systems are configured to perform measurements on the level of light scattered by particles at 90 or 180 degrees to the excitation source, two or more measurements of fluorescence used to determine classification, which is the particle “identity,” and additional fluorescence measurements known as “reporters,” typically used to quantify chemical reactions of interest. Each of the fluorescent measurements is made at different wavelengths.
One excitation laser commonly used in flow cytometers is a 532 nm solid-state laser. Such a laser tends to have a relatively large beam diameter (e.g., about 0.3 mm). A lens system may be used to reduce the beam diameter of the laser to an elliptical spot having lateral dimensions of about 75 μm by about 25 μm. The elliptical spot lies within an optical sensor's detection window. There are, however, several disadvantages to the 532 nm laser. For example, the 532 nm laser is quite expensive (e.g., about $5,500 each), consumes significant electrical power, and generates a substantial amount of heat.
Another laser that is used in commercially available flow cytometers is an argon ion 488 nm laser. There are, however, also several disadvantages to this laser. For example, it is relatively large (e.g., occupying several cubic feet), requires a massive power supply, and needs constant forced air cooling to maintain stability. There are other smaller and less expensive lasers that are commercially available. However, these lasers are generally unsuitable for flow cytometry. For example, dye lasers may burn out too quickly to be used as suitable light sources in a flow cytometer based measurement system. In addition, He—Cd lasers may be too noisy for flow cytometer measurements.
Furthermore, the beam profile of a laser diode may be relatively uneven compared to that of a standard argon ion laser. The unevenness presents a significant obstacle for flow analyzers because fluorescence measurements depend upon substantially uniform excitation among particles and cells. Some efforts have been made to optically correct the beam by steering outside peaks in the beam profile toward the center using beam shaping optics such as prismatic expanders, beam shaping expanders, and micro lens arrays. However, such optics are relatively expensive and add to the manufacturing complexity of the flow cytometers. In addition, even when expensive and complex beam shaping optics are used, the resulting beam profile may still be unsatisfactory (e.g., a 10% to 15% variation in energy intensity across the flow path).
Accordingly, it may be advantageous to provide an excitation source for a flow cytometry based measurement system that is less expensive, consumes less power, generates less heat, is smaller in size, has a longer lifetime, is less noisy, and/or is less weak than the lasers mentioned above. The excitation source also preferably has a wavelength that is suitable for flow cytometer type measurements.